As an example of a conventional fluid machine having an expansion mechanism and a compression mechanism, an expander-compressor unit is disclosed in JP 2005-299632 A. As shown in FIG. 18, an expander-compressor unit 130 described in JP 2005-299632 A includes a closed casing 120, a compression mechanism 121, a motor 122, and an expansion mechanism 123. The motor 122, the compression mechanism 121 and the expansion mechanism 123 are coupled to each other by a mechanical power recovery shaft 124. The expansion mechanism 123 recovers mechanical power from an expanding refrigerant. The mechanical power recovered by the expansion mechanism 123 is applied to the compression mechanism 121 via the mechanical power recovery shaft 124. Thereby, the power consumption of the motor 122 for driving the compression mechanism 121 is reduced. As a result, the coefficient of performance (COP) of a refrigeration cycle apparatus using this expander-compressor unit 130 is improved.
In the expander-compressor unit 130, a bottom portion 125 of the closed casing 120 is used as an oil reservoir for containing refrigerating machine oil. The refrigerating machine oil contained in the bottom portion 125 is pumped up to the upper part of the closed casing 120 by an oil pump 126 disposed at the lower end portion of the mechanical power recovery shaft 124. The refrigerating machine oil pumped up by the oil pump 126 is supplied, through an oil supply passage 127 formed inside the mechanical power recovery shaft 124, to the compression mechanism 121 and the expansion mechanism 123. Thereby, lubrication and sealing of the sliding parts of the compression mechanism 121 and the expansion mechanism 123 are ensured.
An oil return passage 128 is formed at an upper part of the expansion mechanism 123. One end of the oil return passage 128 is connected to the oil supply passage 127 in the mechanical power recovery shaft 124. The other end of the oil return passage 128 opens downwardly to the space below the expansion mechanism 123. Generally, in order to ensure the reliability of the expansion mechanism 123, an excess amount of refrigerating machine oil is supplied to the expansion mechanism 123. The excess refrigerating machine oil is returned to the oil reservoir through the above-mentioned oil return passage 128.
The amount of refrigerating machine oil mixed in the refrigerant and discharged from the compression mechanism 121 together with the refrigerant is different from the amount of refrigerating machine oil mixed in the refrigerant and discharged from the expansion mechanism 123. Therefore, in the case where the compression mechanism 121 and the expansion mechanism 123 are accommodated in separate closed casings, an excess or shortage of refrigerating machine oil to be contained may occur in the closed casing in which the compression mechanism 121 is accommodated or in the closed casing in which the expansion mechanism 123 is accommodated.
In contrast, in the expander-compressor unit 130, the expansion mechanism 123 and the compression mechanism 121 are disposed in the same closed casing 120, in which they share the common oil reservoir. Therefore, in the expander-compressor unit 130, there arises no problem such as the above-mentioned excess or shortage of oil.
However, like the expander-compressor unit 130, in the case where the expansion mechanism 123 and the compression mechanism 121 are accommodated in the same closed casing 120, heat transfer occurs easily between the expansion mechanism 123 and the compression mechanism 121. When heat transfer occurs between the expansion mechanism 123 and the compression mechanism 121, there arises a problem that the COP of the expander-compressor unit 130 decreases.